1. Field of the Invention
The present invention relates to an apparatus for influencing the frictional conditions between an upper roll and a lower roll of a roll stand of a hot-rolling train for wide strip or a cold-rolling train for strip.
The present invention also relates to a method for rolling a rolling material by influencing the frictional conditions between an upper roll and a lower roll of a roll stand.
2. Description of the Related Art
It has been found in practice that during the rolling operation the torques significantly vary between the upper roll and the lower roll of a roll stand of a hot rolling train for wide strip or a cold rolling for strip. Torque differences between the upper and lower rolls in the order of magnitude of between 30% and 70% are not infrequent.
The reason for these differences are the different frictional conditions in the roll gap, i.e., between the upper roll and the upper strip surface of the rolling stock as well as between the lower roll and the lower strip surface of the rolling stock. The effect of these frictional conditions is that different torque requirements prevail at the upper roll and the lower roll. In addition, these different frictional conditions are also influenced by the so-called angle of contact of the hot or cold strip around the upper roll and the lower roll caused by loop lifting devices or guide rolls.
Different frictional conditions in the roll gap and the resulting uneven distribution of the torque requirements of the upper roll and the lower roll not only influence the configuration of the participating components, such as drive spindles, gear assemblies, etc., but they also constitute an impairment of the strip surface quality, of the duration of use of the rolls between roll changes, and of the service life of the rolls caused by increased wear.
Because of the different frictional conditions, it is also not possible to carry out uniformly distributed pass reductions of the hot or cold strip between the upper roll and the lower roll. Since the resistance of the rolling stock against deformation is a combination of the actual resistance to deformation of the material and of the resistance to flow between the roll and the rolling stock, more material is deformed in the case of a low resistance to flow than in the case of a higher resistance to flow; this means that the decrease or the elongation of the rolling stock is greater in that portion of roll gap which has the lower friction value and, thus, the lower resistance to flow of the rolling stock. This, in turn, has the result that the rolling stock has different qualities over its length, and, therefore, frequently does not meet quality requirements.
The pass reduction leads to stretching which means that the rolling stock is elongated. In the entry area of the roll stand, the rolling stock arrives at the two rolls at the upper side as well as on the bottom side of the strip with the same entry speed. Consequently, the rolling stock is essentially forced to leave the rolls with the same speed at the upper side as well as at the lower side of the strip. However, this requires that the friction values between the upper roll and the upper surface of the strip as well as the friction values between the lower roll and the bottom side of the strip are equal. However, as already described above, different friction values occur in practical operation, so that the rolling stock slides between the rolls. This additional relative movement between rolls and rolling stock caused by the different frictional conditions can be recognized by the pattern of wear of the roll surfaces. In addition, the additional relative movement can be recognized by a ski-like shaping of the strip beginning, i.e., after leaving the stand, the strip is bent upwardly or downwardly. In either case, these relative movements occur in addition to the flow of the rolling stock during the deformation, wherein these relative movements impair the efficiency of the rolling process and, thus, not only increase the energy requirement and the wear of the rolls, but they also significantly impair the surface quality of the strip.